Line the tin. Cut two strips of parchment paper and lay them crossways in the tin so the ends hang over the sides. This sling is what lets you lift the whole slab out later without wrestling with it.

Crush the biscuits. Pulse 300 g of Oreos in a food processor until they look like dark, damp sand. You want a few small pebbles left — that texture keeps the base from turning to paste. If you are doing this by hand, seal them in a freezer bag and bash them with a rolling pin until you hear fine crumbs rather than big chunks.

Make the base. Tip the crumbs into a bowl, pour over the melted butter, and stir until every crumb looks glossy and slightly clumped. Press the mixture firmly into the lined tin using the base of a measuring cup. The surface should feel solid and level, not loose. Chill for 15 minutes while you make the filling.

Beat the cream cheese. In your mixer, beat the cream cheese and caster sugar on medium speed for 2 minutes. The mixture should turn smooth, pale, and silky, with no visible lumps. Scrape down the bowl halfway through — there is always a pocket of uncreamed cheese hiding at the bottom.

Whip the cream. In a separate bowl, whisk the cold double cream, icing sugar, and vanilla until soft peaks form. Lift the beaters and the cream should hold a gentle curl rather than dripping off. Overwhipping gives you a grainy texture, so stop early if in doubt.

Fold everything together. Add the lemon juice and salt to the cream cheese, then fold in the whipped cream in three additions. Use a spatula and a gentle cutting motion. The mixture should look uniform, thick, and glossy, with no streaks of white cream remaining.

Add the biscuit pieces. Roughly chop the 6 extra Oreos and fold them through with two or three turns. You want visible rubble, not a grey mush.

Assemble the bars. Spoon the filling over the chilled base and smooth the top with an offset knife. Tap the tin twice on the counter to knock out air bubbles. Scatter a few crushed Oreos over the surface and press them in lightly with your fingertips.

Chill. Cover with cling film and refrigerate for at least 4 hours, ideally overnight. The bars are ready when the surface feels firm and cool and the filling does not wobble when you nudge the tin.

Slice and serve. Lift the slab out using the parchment sling, then cut into 12 bars with a hot, dry knife, wiping between cuts. The edges should be clean and sharp, and each slice should hold its shape on the plate.